Catalysts for treating transient NOx emissions

ABSTRACT

A heterogeneous catalyst article having at least one combination of a first molecular sieve having a medium pore, large pore, or meso-pore crystal structure and optionally containing a first metal, and a second molecular sieve having a small pore crystal structure and optionally containing a second metal, and a monolith substrate onto or within which said catalytic component is incorporated, wherein the combination of the first and second molecular sieves is a blend, a plurality of layers, and/or a plurality of zones.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/746,074 (now U.S. Pat. No. 9,616,420), filedJan. 21, 2013, which is a divisional of U.S. patent application Ser. No.13/477,305 filed May 22, 2012 which is a continuation of PCTInternational Application No. PCTIB2010003186, filed on Nov. 30, 2010,and claims priority benefit to Great Britain Patent Application No.0920927.1 filed on Nov. 30, 2009, the disclosures of each of which areincorporated herein by reference.

The present invention relates to a selective catalytic reductioncatalyst comprising an optionally metal-promoted molecular sievecomponent for converting oxides of nitrogen (NO_(x)) present in exhaustgas emitted from a mobile source, such as a vehicular lean-burninternal-combustion engine, in the presence of a nitrogenous reductant.

As used herein, the term “selective catalytic reduction” (SCR) definesthe catalytic process of reducing oxides of nitrogen to dinitrogen (N₂)using a nitrogenous reductant. SCR is known from treating NO_(x)emissions from industrial stationary source applications, such asthermal power plants. More recently the SCR technique has been developedfor treating NO_(x) emissions from mobile source applications, such aspassenger cars, trucks and buses. A difficulty in treating NO_(x) frommobile source applications is that the quantity of NO_(x) present in theexhaust gas is transient, i.e. it varies with driving conditions, suchas acceleration, deceleration and cruising at various speeds. Thetransient nature of the NO_(x) component in the mobile applicationexhaust gas presents a number of technical challenges, including correctmetering of nitrogenous reductant to reduce sufficient NO_(x) withoutwaste or emission of nitrogenous reductant to atmosphere.

In practice, SCR catalysts can adsorb (or store) nitrogenous reductant,thus providing a buffer to the appropriate supply of availablereductant. Technologists use this phenomenon to calibrate appropriatenitrogenous reductant injection to exhaust gas.

So, in summary, SCR catalysts for mobile source applications broadlyperform three functions: (i) convert NO_(x) using e.g. ammonia (NH₃) asnitrogenous reductant; (ii) store the NH₃ when there is excess NH₃ inthe gas feed; and (iii) utilise the stored NH₃ under conditions wherethere is not sufficient NH₃ present in the gas feed to achieve therequired conversion.

For practical applications, like treating NO_(x) emissions from a mobileNO_(x) source, such as a motor vehicle, where the feed gas conditionsare rapidly changing, a desirable SCR catalyst has sufficient NH₃storage capacity at a given temperature (to ensure any excess NH₃ is not“slipped” past the catalyst and to allow conversion to continue if NH₃is not present in the feed) and high activity independent of thefraction of NH₃ fill level (fill level is defined relative to asaturated NH₃ storage capacity). The NH₃ fill level can be expressed asthe amount of NH₃ (for example in grams) present on the completecatalyst (for example in liters) relative to a maximum fill level at agiven set of conditions. NH₃ adsorption can be determined according tomethods known in the art, such as Langmuir absorption. It will beunderstood that the fill level of all SCR catalysts is not directlyproportional to the maximal NO_(x) conversion activity of the SCRcatalyst, i.e. it does not follow that NO_(x) conversion activityincreases to a maximum at 100% ammonia fill level. In fact, specific SCRcatalysts can show maximal NO_(x) conversion rates at a fill level of<100%, such as <90%, <80%, <50% or <30%.

The activity of a SCR catalyst can depend on the amount of NH₃ to whichthe entire catalyst monolith has been exposed. Molecular sieve-based SCRcatalysts can store ammonia, and the amount of storage capacity depends,among others, on the temperature of the gas stream and the catalyst, thefeed gas composition, the space velocity, particularly the NO:NO₂ ratioetc. The catalyst activity at the onset of exposure of the catalyst toNH₃ can be substantially lower than the activity when the catalyst has arelatively high exposure or saturated exposure to NH₃. For practicalvehicle applications, this means the catalyst needs to be pre-loadedwith an appropriate NH₃ loading to ensure good activity. However, thisrequirement presents some significant problems. In particular, for someoperating conditions, it is not possible to achieve the required NH₃loading; and this pre-loading method has limitations because it is notpossible to know what the engine operating conditions will be subsequentto pre-loading. For example, if the catalyst is pre-loaded with NH₃ butthe subsequent engine load is at idle, NH₃ may be slipped to atmosphere.Hence, in practical applications the amount of NH₃ pre-stored has to belower than is optimal to ensure that there is limited slip of NH₃ if theengine is operated in a high load condition that needs NH₃ pre-loadinginstead of a lower load condition.

SCR catalysts for use on mobile applications such as automotive, arerequired to operate at low temperature whilst also being tolerant tohydrocarbons. Low temperature operation usually means that there is verylittle NO₂ in the feed gas, which favours the use of copper-based SCRcatalysts. However, iron-based SCR catalysts are typically very good attreating approximately 50:50 NO:NO₂ gas feeds and are also good underhigh temperature conditions that may be experienced should an exhaustsystem contain a catalysed soot filter (CSF) and the system is arrangedso that the CSF is regenerated (i.e. collected particulate matter iscombusted) periodically by engineering forced high temperatureconditions.

WO 2008/132452 discloses a method of converting nitrogen oxides in agas, such as an exhaust gas of a vehicular lean-burn internal combustionengine, to nitrogen by contacting the nitrogen oxides with a nitrogenousreducing agent in the presence of a molecular sieve catalyst containingat least one transition metal, wherein the molecular sieve is a smallpore zeolite containing a maximum ring size of eight tetrahedral atoms,wherein the at least one metal is selected from the group consisting ofCr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Irand Pt. Suitable small pore molecular sieves include (using thethree-letter code recognised by the Structure Commission of theInternational Zeolite Association) CHA, including SSZ-13 and SAPO-34;LEV, such as Nu-3; DDR e.g. Sigma-1; and ERI, including ZSM-34. Broadly,we have found that SCR catalysts for use in the method of WO 2008/132452show a maximum NO_(x) conversion at relatively high fill level.

WO '452 explains certain drawbacks to using ZSM-5 and Beta zeolites forconverting NO_(x) in exhaust gases emitted by mobile sources, such asvehicles, including that they are susceptible to dealumination duringhigh temperature hydrothermal ageing resulting in a loss of acidity,especially with Cu/Beta and Cu/ZSM-5 catalysts; both Beta- andZSM-5-based catalysts are also affected by hydrocarbons which becomeadsorbed on the catalysts at relatively low temperatures (known as“coking”) which hydrocarbons can be oxidised subsequently as thetemperature of the catalytic system is raised generating a significantexotherm, which can thermally damage the catalyst. This problem isparticularly acute in vehicular diesel applications where significantquantities of hydrocarbon can be adsorbed on the catalyst duringcold-start. Coking can also reduce catalytic activity because activecatalyst sites can become blocked.

According to WO '452, transition metal-containing small pore molecularsieve-based SCR catalysts demonstrate significantly improved NO_(x)reduction activity than the equivalent transition metal-containingmedium, large or meso-pore molecular sieve catalysts, transitionmetal-containing small pore molecular sieve catalysts, especially at lowtemperatures. They also exhibit high selectivity to N₂ (e.g. low N₂Oformation) and good hydrothermal stability. Furthermore, small poremolecular sieves containing at least one transition metal are moreresistant to hydrocarbon inhibition than larger pore molecular sieves.

During testing of certain SCR catalysts disclosed in WO 2008/132452 forreducing NO_(x) with nitrogenous reductants (urea, an NH₃ precursor) itwas discovered that the transient response of the catalysts wassub-optimal for treating NO_(x) in transient vehicular exhaust gas. Thatis, the ability of the SCR catalysts to treat NO_(x) in the transientlychanging exhaust gas composition was less than desirable.

SAE 2008-01-1185 discloses a selective catalytic reduction catalystcomprising separate iron zeolite and copper zeolite catalysts arrangedin zones coated one behind the other on a flow-through substratemonolith with the iron zeolite zone disposed upstream of the copperzeolite zone. No details are given regarding the zeolites used. Resultsfor transient response (shown in FIG. 17) for the combined ironzeolite/copper zeolite catalyst compared unfavourably to the use ofcopper zeolite alone.

We have now discovered, very surprisingly, that combinations oftransition metal/molecular sieve, e.g. zeolite, catalysts are moreactive for NO_(x) conversion but also have relatively fast transientresponse. We have also found that combinations of iron molecular sievecatalysts can give good activity as well as being hydrocarbon tolerant.

According to one aspect, the invention provides a heterogeneous catalystarticle comprising (a) a catalytic component comprising a combination ofa first molecular sieve having a medium pore, large pore, or meso-porecrystal structure and optionally containing about 0.01 to about 20weight percent of a first metal, and a second molecular sieve having asmall pore crystal structure and optionally containing about 0.01 toabout 20 weight percent of a second metal, wherein said first and secondmetals are exchanged or free with respect to the molecular sieve'scrystalline frame work and are independently selected from the groupconsisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Pt,Ag, In, Sn, Re, and Ir; and (b) a monolith substrate onto or withinwhich said catalytic component is incorporated, wherein said combinationof the first and second molecular sieves is selected from the groupconsisting of a blends, a plurality of layers, and a plurality of zones.

Types of combinations of catalysts that are useful in the presentinvention include blends of two or more catalysts, a plurality of layerswherein each layer consisting of a single catalyst, and a plurality ofzones, wherein each zone consists of a single catalyst. The combinationsare characterized by properties that are not obtainable by any of theirconstituent parts acting independently of the combination. Turning toFIGS. 4a-4d , shown are certain embodiments of different combinationsaccording to the present invention. FIG. 4a shows of a blend 100comprising a blend of two molecular sieves 104 coated on a substrate102. As used herein, the term “blend”, with respect to molecular sievesmeans a volume of two or more molecular sieves having approximately thesame proportions relative to one another throughout the volume. Alsoshown is an embodiment of a plurality of zones 110 comprising a firstzone consisting of a first molecular sieve 116 and a second zonecomprising a second molecular sieve 118, wherein the first and secondzones are coated on a substrate 102 and are adjacent to each other andto said monolith substrate. The direction 114 of exhaust gas flow isalso shown. In FIG. 4c , shown is an embodiment of a plurality of layers120 comprising first layer 116 and a second layer 118, wherein thesecond layer is adjacent to both said first layer and the substrate andis between the first layer and the substrate. FIG. 4d shows twocombinations 130, wherein the first combination is two zones (molecularsieve 116 and blend 104) and the second combination is blend 104. Forembodiments that utilize two or more combinations, the molecular sievesfor each combination are independently selected. Although not shown inthe figures, other multiple combinations are within the scope of theinvention as well. For example, an arrangement similar to that of FIG.4d , but instead of a blend, the second combination could be a pluralityof layers. Other multiple combinations include the use of a blend as oneor more layers; the use of layers as one or more zones; and the like.When multiple combinations are used, the order of combinations withrespect to exhaust gas flow through the catalyst component is notparticularly limited. However, it is highly preferred that at least onemedium, large, or meso-pore molecular sieve always be disposed upstreamof any small pore molecular sieves.

The combinations preferably have a majority of the first molecular sievecomponent relative to the second molecular sieve component. In certainembodiments, combination comprises the first molecular sieve and thesecond molecular sieve in a first molecular sieve: second molecularsieve weight ratio of about 0.1 (i.e., 1:10) to about 1 (i.e., (1:1). Incertain embodiments, the weight ratio of first molecular sieve to secondmolecular sieve is about 0.25 to about 0.50. In certain embodiments, theweight ratio of first molecular sieve to second molecular sieve is about0.3 to about 0.4.

According to another aspect provided is a catalyst for selectivelycatalysing the conversion of oxides of nitrogen using a nitrogenousreductant in a feed gas whose composition, flow rate and temperature areeach changeable temporally, which catalyst comprising a combination of afirst molecular sieve component and a second molecular sieve component,wherein in a direct comparison tested on the Federal Test Procedure(FTP) 75 cycle the catalyst has a higher cumulative conversion of NO_(x)at equal or lower NH₃ slip than either molecular sieve component takenalone.

In particular, we have observed in at least one embodiment a synergicrelationship between the first molecular sieve component and the secondmolecular sieve component which can be used to improve a transientresponse to NO_(x) conversion of a SCR catalyst comprising molecularsieve, e.g. a small pore molecular sieve while retaining the advantagesof using the small pore molecular sieve as a component in a SCRcatalyst. A “catalyst for selectively catalysing the reduction of oxidesof nitrogen in a feed gas with a nitrogenous reductant” shall bereferred to herein as a “selective catalytic reduction” (or “SCR”)catalyst. For the avoidance of doubt, it is intended that SCR catalystscontaining combinations of three or more molecular sieves fall withinthe scope of the present invention.

In a preferred embodiment, the catalyst has a higher cumulativeconversion of NO_(x) at equal or lower NH₃ slip than either molecularsieve component taken alone where the cumulative molar NO:NO₂ ratio infeed gas entering said catalyst is equal to or less than 1. In certainother preferred embodiments, the NO:NO₂ ratio in feed exhaust gas streamis about 0.8 to about 1.2. In certain other preferred embodiments, theNO:NO₂ ratio in feed exhaust gas stream is less than about 0.3, while inother preferred embodiments, the ratio is greater than about 3.

In a further preferred embodiment, the catalyst has a higher cumulativeconversion of NO_(x) to dinitrogen at equal or lower NH₃ slip thaneither molecular sieve component taken alone.

This invention significantly improves catalyst activity so that higheractivity is obtained at lower NH₃ exposures (low exposure relative tothe saturated storage capacity of the catalyst) compared to currentstate-of-the-art SCR catalysts. The rate of increase of activity fromzero ammonia exposure to saturated ammonia exposure is referred to asthe ‘transient response’.

In one embodiment, the first molecular sieve component achieves themaximum NO_(x) conversion at a lower NH₃ fill level for the conditionsselected than the second molecular sieve component. For example, theammonia fill level of the first molecular sieve component can be in therange of 10-80%, such as 20-60% or 30-50%.

The first and second molecular sieves can be selected independently fromzeolites and non-zeolite molecular sieves. “Zeolite” according to theInternational Zeolite Association, is generally considered to be analumino-silicate, whereas a “non-zeolite molecular sieve” can be amolecular sieve of the same Framework Type (or crystal structure) as thecorresponding zeolite, but having one or more non-aluminium/non-siliconcations present in its crystal lattice, e.g. phosphorus, both cobalt andphosphorus, copper or iron. So, for example, SSZ-13 is a zeolite ofFramework Type Code CHA, whereas SAPO-34 is a silico-aluminophosphatenon-zeolite molecular sieve sharing the same CHA Framework Type Code.Particularly preferred are iron-containing aluminosilicate zeolites(non-zeolite molecular sieves as defined herein) such as Fe-containingZSMS, Beta, CHA or FER disclosed for example in WO2009/023202 andEP2072128A1, which are hydrothermally stable and have relatively highSCR activity. Advantageously we have also found that in certainembodiments catalysts comprising these iron-containing aluminosilicatezeolites produce little or no ammonium nitrate, and exhibit relativelyhigh selectivity, e.g. low N₂O. Typical SiO₂/Al₂O₃ mole ratios for suchmaterials are 30 to 100 and SiO₂/Fe₂O₃ of 20 to 300 such as 20 to 100.

In preferred embodiments, the first (zeolitic or non-zeolitic) molecularsieve component can be a small pore molecular sieve containing a maximumring size of eight (8) tetrahedral atoms, optionally selected from anyset out in Table 1. Optionally, the second (zeolitic or non-zeolitic)molecular sieve component also can be a small pore molecular sievecontaining a maximum ring size of eight (8) tetrahedral atoms and can beselected independently of the first molecular sieve component from anyset out in Table 1.

TABLE 1 Details of small pore molecular sieves with application in thepresent invention Framework Type (by Type material* and Frameworkillustrative isotypic Dimen- Type Code) framework structures sionalityPore size (Å) Additional info ACO *ACP-1 3D 3.5 × 2.8, 3.5 × Ringsizes - 8, 4 3.5 AEI *AlPO-18 3D 3.8 × 3.8 Ring sizes - 8, 6, 4[Co—Al—P—O]-AEI SAPO-18 SIZ-8 SSZ-39 AEN *AlPO-EN3 2D 4.3 × 3.1, 2.7 ×Ring sizes - 8, 6, 4 5.0 AlPO-53(A) AlPO-53(B) [Ga—P—O]-AEN CFSAPO-1ACoIST-2 IST-2 JDF-2 MCS-1 MnAPO-14 Mu-10 UiO-12-500 UiO-12-as AFN*AlPO-14 3D 1.9 × 4.6, 2.1 × Ring sizes - 8, 6, 4 4.9, 3.3 × 4.0|(C₃N₂H₁₂)—|[Mn—Al—P—O]- AFN GaPO-14 AFT *AlPO-52 3D 3.8 × 3.2, 3.8 ×Ring sizes - 8, 6, 4 3.6 AFX *SAPO-56 3D 3.4 × 3.6 Ring sizes - 8, 6, 4MAPSO-56, M═Co, Mn, Zr SSZ-16 ANA *Analcime 3D 4.2 × 1.6 Ring sizes - 8,6, 4 AlPO₄-pollucite AlPO-24 Ammonioleucite [Al—Co—P—O]-ANA[Al—Si—P—O]-ANA |Cs—|[Al—Ge—O]-ANA |Cs—|[Be—Si—O]-ANA|Cs₁₆|[Cu₈Si₄₀O₉₆]- ANA |Cs—Fe|[Si—O]-ANA |Cs—Na—(H₂O)|[Ga—Si—O]- ANA[Ga—Ge—O]-ANA |K—|[B—Si—O]-ANA |K—|[Be—B—P—O]-ANA |Li—|[Li—Zn—Si—O]-ANA|Li—Na|[Al—Si—O]-ANA |Na—|[Be—B—P—O]- ANA |(NH₄)—|[Be—B—P—O]- ANA|(NH₄)—|[Zn—Ga—P—O]- ANA [Zn—As—O]-ANA Ca-D Hsianghualite Leucite Na—BPollucite Wairakite APC *AlPO—C 2D 3.7 × 3.4, 4.7 × Ring sizes - 8, 6, 42.0 AlPO—H3 CoAPO—H3 APD *AlPO-D 2D 6.0 × 2.3, 5.8 × Ring sizes - 8, 6,4 1.3 APO—CJ3 ATT *AlPO-12-TAMU 2D 4.6 × 4.2, 3.8 × Ring sizes - 8, 6, 43.8 AlPO-33 RMA-3 CDO *CDS-1 2D 4.7 × 3.1, 4.2 × Ring sizes - 8, 5 2.5MCM-65 UZM-25 CHA *Chabazite 3D 3.8 × 3.8 Ring sizes - 8, 6, 4 AlPO-34[Al—As—O]-CHA [Al—Co—P—O]-CHA |Co| [Be—P—O]-CHA |Co₃ (C₆N₄H₂₄)₃ (H₂O)₉|[Be₁₈P₁₈O₇₂]- CHA [Co—Al—P—O]-CHA |Li—Na| [Al—Si—O]- CHA [Mg—Al—P—O]-CHA[Si—O]-CHA [Zn—Al—P—O]-CHA [Zn—As—O]-CHA CoAPO-44 CoAPO-47 DAF-5 GaPO-34K-Chabazite Linde D Linde R LZ-218 MeAPO-47 MeAPSO-47 (Ni(deta)₂)-UT-6Phi SAPO-34 SAPO-47 SSZ-13 UiO-21 Willhendersonite ZK-14 ZYT-6 DDR*Deca-dodecasil 3R 2D 4.4 × 3.6 Ring sizes - 8, 6, 5, 4 [B—Si—O]-DDRSigma-1 ZSM-58 DFT *DAF-2 3D 4.1 × 4.1, 4.7 × Ring sizes - 8, 6, 4 1.8ACP-3, [Co—Al—P—O]- DFT [Fe—Zn—P—O]-DFT [Zn—Co—P—O]-DFT UCSB—3GaGeUCSB—3ZnAs UiO-20, [Mg—P—O]- DFT EAB *TMA-E 2D 5.1 × 3.7 Ring sizes - 8,6, 4 Bellbergite EDI *Edingtonite 3D 2.8 × 3.8, 3.1 × Ring sizes - 8, 42.0 |(C₃H₁₂N₂)_(2.5)| [Zn₅P₅O₂₀]-EDI [Co—Al—P—O]-EDI [Co—Ga—P—O]-EDI|Li—|[Al—Si—O]-EDI |Rb₇ Na (H₂O)₃| [Ga₈Si₁₂O₄₀]-EDI [Zn—As—O]-EDI K—FLinde F Zeolite N EPI *Epistilbite 2D 4.5 × 3.7, 3.6 × Ring sizes - 8, 43.6 ERI *Erionite 3D 3.6 × 5.1 Ring sizes - 8, 6, 4 AlPO-17 Linde TLZ-220 SAPO-17 ZSM-34 GIS *Gismondine 3D 4.5 × 3.1, 4.8 × Ring sizes -8, 4 2.8 Amicite [Al—Co—P—O]-GIS [Al—Ge—O]-GIS [Al—P—O]-GIS [Be—P—O]-GIS|(C₃H₁₂N₂)₄| [Be₈P₈O₃₂]-GIS |(C₃H₁₂N₂)₄| [Zn₈P₈O₃₂]-GIS [Co—Al—P—O]-GIS[Co—Ga—P—O]-GIS [Co—P—O]-GIS |Cs₄|[Zn₄B₄P₈O₃₂]- GIS [Ga—Si—O]-GIS[Mg—Al—P—O]-GIS |(NH₄)₄|[Zn₄B₄P₈O₃₂]- GIS |Rb₄|[Zn₄B₄P₈O₃₂]- GIS[Zn—Al—As—O]-GIS [Zn—Co—B—P—O]-GIS [Zn—Ga—As—O]-GIS [Zn—Ga—P—O]-GISGarronite Gobbinsite MAPO-43 MAPSO-43 Na—P1 Na—P2 SAPO-43 TMA-gismondineGOO *Goosecreekite 3D 2.8 × 4.0, 2.7 × Ring sizes - 8, 6, 4 4.1, 4.7 ×2.9 IHW *ITQ-32 2D 3.5 × 4.3 Ring sizes - 8, 6, 5, 4 ITE *ITQ-3 2D 4.3 ×3.8, 2.7 × Ring sizes - 8, 6, 5, 4 5.8 Mu-14 SSZ-36 ITW *ITQ-12 2D 5.4 ×2.4, 3.9 × Ring sizes - 8, 6, 5, 4 4.2 LEV *Levyne 2D 3.6 × 4.8 Ringsizes - 8, 6, 4 AlPO-35 CoDAF-4 LZ-132 NU-3 RUB-1 [B—Si—O]-LEV SAPO-35ZK-20 ZnAPO-35 KFI ZK-5 3D 3.9 × 3.9 Ring sizes - 8, 6, 4|18-crown-6|[Al—Si—O]- KFI [Zn—Ga—As—O]-KFI (Cs,K)—ZK-5 P Q MER*Merlinoite 3D 3.5 × 3.1, 3.6 × Ring sizes - 8, 4 2.7, 5.1 × 3.4, 3.3 ×3.3 [Al—Co—P—O]-MER |Ba—|[Al—Si—O]-MER |Ba—Cl—|[Al—Si—O]- MER[Ga—Al—Si—O]-MER |K—|[Al—Si—O]-MER |NH₄—|[Be—P—O]-MER K—M Linde WZeolite W MON *Montesommaite 2D 4.4 × 3.2, 3.6 × Ring sizes - 8, 5, 43.6 [Al—Ge—O]-MON NSI *Nu-6(2) 2D 2.6 × 4.5, 2.4 × Ring sizes - 8, 6, 54.8 EU-20 OWE *UiO-28 2D 4.0 × 3.5, 4.8 × Ring sizes - 8, 6, 4 3.2 ACP-2PAU *Paulingite 3D 3.6 × 3.6 Ring sizes - 8, 6, 4 [Ga—Si—O]-PAU ECR-18PHI *Phillipsite 3D 3.8 × 3.8, 3.0 × Ring sizes - 8, 4 4.3, 3.3 × 3.2[Al—Co—P—O]-PHI DAF-8 Harmotome Wellsite ZK-19 RHO *Rho 3D 3.6 × 3.6Ring sizes - 8, 6, 4 [Be—As—O]-RHO [Be—P—O]-RHO [Co—Al—P—O]-RHO|H—|[Al—Si—O]-RHO [Mg—Al—P—O]-RHO [Mn—Al—P—O]-RHO |Na₁₆ Cs₈|[Al₂₄Ge₂₄O₉₆]-RHO |NH₄—|[Al—Si—O]-RHO |Rb—|[Be—As—O]-RHO GallosilicateECR-10 LZ-214 Pahasapaite RTH *RUB-13 2D 4.1 × 3.8, 5.6 × Ring sizes -8, 6, 5, 4 2.5 SSZ-36 SSZ-50 SAT *STA-2 3D 5.5 × 3.0 Ring sizes - 8, 6,4 SAV *Mg-STA-7 3D 3.8 × 3.8, 3.9 × Ring sizes - 8, 6, 4 3.9 Co-STA-7Zn-STA-7 SBN *UCSB-9 3D TBC Ring sizes - 8, 4, 3 SU-46 SIV *SIZ-7 3D 3.5× 3.9, 3.7 × Ring sizes - 8, 4 3.8, 3.8 × 3.9 THO *Thomsonite 3D 2.3 ×3.9, 4.0 × Ring sizes - 8, 4 2.2, 3.0 × 2.2 [Al—Co—P—O]-THO[Ga—Co—P—O]-THO |Rb₂₀|[Ga₂₀Ge₂₀O₈₀]- THO [Zn—Al—As—O]-THO [Zn—P—O]-THO[Ga—Si—O]-THO) [Zn—Co—P—O]-THO TSC *Tschörtnerite 3D 4.2 × 4.2, 5.6 ×Ring sizes - 8, 6, 4 3.1 UEI *Mu-18 2D 3.5 × 4.6, 3.6 × Ring sizes - 8,6, 4 2.5 UFI *UZM-5 2D 3.6 × 4.4, 3.2 × Ring sizes - 8, 6, 4 3.2 (cage)VNI *VPI-9 3D 3.5 × 3.6, 3.1 × Ring sizes - 8, 5, 4, 3 4.0 YUG*Yugawaralite 2D 2.8 × 3.6, 3.1 × Ring sizes - 8, 5, 4 5.0 Sr-Q ZON*ZAPO-M1 2D 2.5 × 5.1, 3.7 × Ring sizes - 8, 6, 4 4.4 GaPO-DAB-2 UiO-7

In one embodiment, the small pore molecular sieves can be selected fromthe group of Framework Type Codes consisting of: ACO, AEI, AEN, AFN,AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI,GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO,RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON.

Small pore molecular sieves with particular application for treatingNO_(x) in exhaust gases of lean-burn internal combustion engines, e.g.vehicular exhaust gases are set out in Table 2.

TABLE 2 Preferred small pore molecular sieves for use in the SCRcatalyst according to the invention. Structure Molecular Sieve CHASAPO-34 AlPO-34 SSZ-13 LEV Levynite Nu-3 LZ-132 SAPO-35 ZK-20 ERIErionite ZSM-34 Linde type T DDR Deca-dodecasil 3R Sigma-1 KFI ZK-518-crown-6 [Zn—Ga—As—O]-KFI EAB TMA-E PAU ECR-18 MER Merlinoite AEISSZ-39 GOO Goosecreekite YUG Yugawaralite GIS P1 VNI VPI-9

In particular embodiments, the second molecular sieve (either a zeoliteor a non-zeolite molecular sieve) component can be a medium pore, largepore or meso-pore size molecular sieve.

In particularly preferred embodiments, the first molecular sieve is aCuCHA material and the second molecular sieve is a FeBEA, FeFER, FeCHAor FeMFI (e.g. ZSM-5) wherein the Fe is impregnated, ion-exchangedand/or present within the crystal lattice of the molecular sieve.

By “medium pore” herein we mean a molecular sieve containing a maximumring size of ten (10) and by “large pore” herein we mean containing amaximum ring size of twelve (12) tetrahedral atoms. Meso-pore molecularsieves have a maximum ring size of >12.

Suitable medium pore molecular sieves for use as second molecular sievesin the present invention include ZSM-5 (MFI), MCM-22 (MWW), AlPO-11 andSAPO-11 (AEL), AlPO-41 and SAPO-41 (AFO), ferrierite (FER), Heulanditeor Clinoptilolite (HEU). Large pore molecular sieves for use in thepresent invention include zeolite Y, such as ultrastable-Y (or USY),faujasite or SAPO-37 (FAU), AlPO-5 and SAPO-5 (AFI), SAPO-40 (AFR),AlPO-31 and SAPO-31 (ATO), Beta (BEA), Gmelinite (GME), mordenite (MOR)and Offretite (OFF).

It will be appreciated from comments in WO 2008/132452 that the use ofcertain medium and large pore molecular sieves, such as ZSM-5 zeolite orBeta zeolite, can result in catalyst coking. Selection of certain mediumand large pore molecular sieve components may be inappropriate for someapplications: essentially a balance is being struck between improvedtransient response on the one hand and coking issues on the other.However, it may be possible to reduce or avoid such coking problems withappropriate exhaust system design, e.g. location of an oxidationcatalyst upstream of the SCR catalyst which can convert somehydrocarbons in the feed gas that could otherwise have coked medium,large or meso-pore molecular sieve components. It is also possible incertain embodiments where a small pore molecular sieve is combined witha medium, large or meso-pore molecular sieve that the presence of thesmall pore molecular sieve reduces the coking on the medium, large ormeso-pore molecular sieve. Another benefit of this arrangement is that aratio of NO:NO₂ in feed gas contacting the catalyst can be adjusted toimprove total NO_(x) conversion on the SCR catalyst.

Molecular sieves for use in the present invention can be independentlyselected from one-dimensional, two-dimensional and three-dimensionalmolecular sieves. Molecular sieves showing three-dimensionaldimensionality have a pore structure, which is interconnected in allthree crystallographic dimensions, whereas a molecular sieve havingtwo-dimensional dimensionality has pores which are interconnected in twocrystallographic dimensions only. A molecular sieve havingone-dimensional dimensionality has no pores that are interconnected froma second crystallographic dimension.

Small pore molecular sieves, particularly aluminosilicate zeolites, foruse in the present invention can have a silica-to-alumina ratio (SAR) offrom 2 to 300, optionally 4 to 200 such as 8 to 150 e.g. 15 to 50 or 25to 40. It will be appreciated that higher SARs are preferred to improvethermal stability (especially high catalytic activity at a lowtemperature after hydrothermal ageing) but this may negatively affecttransition metal exchange. Therefore, in selecting preferred materialsconsideration can be given to SAR so that a balance may be struckbetween these two properties. SAR for iron-in-framework molecular sievesis discussed elsewhere in this description.

In preferred embodiments, the first molecular sieve, the secondmolecular sieve or both the first and second molecular sieves containone or more metal selected independently from the group consisting ofCr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Irand Pt. The metal contained in the first molecular sieve can be the sameor different from that of the second molecular sieve. So for example,the first molecular sieve can contain copper and the second molecularsieve can contain iron. In one embodiment, the two molecular sieves canbe ion-exchanged together.

It will be appreciated, e.g. from Table 1 hereinabove that by “molecularsieves containing one or more transition metal” herein we intend tocover molecular sieves wherein elements other than aluminium and siliconare substituted into the framework of the molecular sieve. Suchmolecular sieves are known as “non-zeolitic molecular sieves” andinclude “SAPO”, “MeAPO”, “FeAPO”, “AlPO₄”, “TAPO”, “ELAPO”, “MeAPSO” and“MeAlPO” which are substituted with one or more metals. Suitablesubstituent metals include one or more of, without limitation, As, B,Be, Co, Fe, Ga, Ge, Li, Mg, Mn, Ti, Zn and Zr. Such non-zeoliticmolecular sieves can in turn be impregnated by suitable metals listedhereinabove, i.e. Cr, Mn, Fe, Co etc. One or both of the first andsecond molecular sieves can contain substituent framework metals. Whereboth the first and the second molecular sieves contain substituentframework metals, the or each substituent metal is selectedindependently from the above list.

In a particular embodiment, the small pore zeolites and non-zeolitemolecular sieves for use in the present invention can be selected fromthe group consisting of aluminosilicate zeolites, metal-substitutedaluminosilicate molecular sieves, such as the preferred iron-containingaluminosilicate zeolites and aluminophosphate molecular sieves.

Aluminophosphate molecular sieves with application in the presentinvention include aluminophosphate (AlPO₄) molecular sieves, metalsubstituted aluminophosphate molecular sieves (MeAlPO),silico-aluminophosphate (SAPO) molecular sieves and metal substitutedsilico-aluminophosphate (MeAPSO) molecular sieves.

A particularly interesting group of molecular sieve components for useeither as a first or a second molecular sieve component are ironsubstituted aluminosilicates, i.e. where iron is present in theframework of the molecular sieve. In the preferred application of theSCR catalysts for use in the present invention, i.e. for treatingexhaust gas from a mobile NO_(x) source, iron-substitutedaluminosilicates are particularly interesting because they producerelatively low or no N₂O, which is a powerful “greenhouse” gas.

In one embodiment, the at least one transition metal is selected fromthe group consisting of Cr, Ce, Mn, Fe, Co, Ni and Cu. In a preferredembodiment, the at least one transition metal is selected from the groupconsisting of Cu, Fe and Ce. In a particular embodiment, the at leastone transition metal consists of Cu. In another particular embodiment,the at least one transition metal consists of Fe. In a furtherparticular embodiment, the at least one transition metal is Cu and/orFe.

The total of the at least one transition metal that can be included inthe at least one transition metal-containing molecular sieve can be from0.01 to 20.00 wt %, based on the total weight of the molecular sievecatalyst containing at least one transition metal. In one embodiment,the total of the at least one transition metal that can be included canbe from 0.1 to 10.0 wt %. In a particular embodiment, the total of theat least one transition metal that can be included is from 0.5 to 5.0 wt%. In preferred embodiments, the transition metal loading is from 2.0 to4.0 wt % and the SAR is 25 to 50 or >40 or >60 e.g. 40<100, 40<70 or60<100, provided that for iron-in-framework molecular sieves theSiO₂/Fe₂O₃ ratio is met (where present the SiO₂/Fe₂O₃ is 50 to 200,preferably 50 to 100).

Transition metals may be incorporated into the molecular sieves for usein the present invention using techniques well known in the art,including liquid-phase exchange or solid-ion exchange or by an incipientwetness process. For manufacture of iron-containing aluminosilicatezeolites see Journal of Catalysis 232(2) 318-334 (2005); EP2072128; andWO2009/023202 and references and search citations therein.

In a particularly preferred embodiment, the catalytic componentcomprises or consists of a combination of a first molecular sieve thatis a large pore molecular sieve and a second molecular sieve that is asmall pore molecular sieve. In certain preferred embodiments, the smallpore molecular sieve has a CHA framework, more preferably a SSZ-13framework, and contains copper. In certain preferred embodiments, thissmall pore molecular sieve is combined with a large pore molecular sievehaving a BEA framework. Preferably, the BEA framework contains eitherexchanged or free iron or is an iron isomorphous BEA molecular structure(also referred to as BEA-type ferrosilicate), with iron isomorphous BEAmolecular structure being particularly preferred.

In certain preferred embodiments, the iron isomorphous BEA molecularstructure is crystalline silicate having (1) an iron-containingBEA-framework structure that has a SiO₂/Fe₂O₃ mol ratio of about 20 toabout 300, and (2) at least 80% of the contained iron is isolated ironions Fe³⁺ in a fresh state and/or log(SiO₂/Al₂O₃) by mol is at leastabout 2. Preferred BEA-type ferrosilicates useful in the presentinvention have a composition represented by following formula:(x+y)M _((2/n))O·xFe₂O₃ ·yAl₂O₃ ·zSiO₂ ·wH₂Owherein n is an atomic value of cation M; x, y, and z represent molfractions of Fe₂O₃, Al₂O₃ and SiO₂, respectively; x+y+z=1; w is a numberof at least 0; z/x is 20 to 300, y may be 0, and optionally z/y is atleast 100.

Preferably, iron-containing BEA-framework structure that has aSiO₂/Fe₂O₃ mol ratio of about 25 to about 300, about 20 to about 150,about 24 to about 150, about 25 to about 100, or about 50 to about 80.The upper limit of log(SiO₂/Al₂O₃) by mol is not particularly limited,provided that the log(SiO₂/Al₂O₃) by mol is at least 2 (i.e., theSiO₂/Al₂O₃ ratio by mol is at least 100). The log(SiO₂/Al₂O₃) by mol ispreferably at least 2.5 (i.e., the SiO₂/Al₂O₃ ratio by mol is at least310), more preferably at least 3 (i.e., the SiO₂/Al₂O₃ ratio by mol isat least 1,000). When the log(SiO₂/Al₂O₃) by mol exceeds 4 (i.e., theSiO₂/Al₂O₃ ratio by mol becomes at least 10,000), the performance fornitrogen oxide reduction is constant at the highest level.

In certain preferred embodiments, the CHA molecular sieve ischaracterized as having a mean crystal size of greater than about 1microns, preferably about 1 to about 5 microns, with about 2 to about 4microns being most preferred. In the BEA-type ferrosilicate, the ironingredient most prominently exhibiting a catalytic activity for thereduction of nitrogen oxides is not agglomerated as Fe₂O₃ but isdispersed as isolated iron ion Fe³⁺ in the framework structure (i.e.,isolated and dispersed in the silicate frame structure or ion exchangesites). The isolated iron ion Fe₃₊ can be detected by the electron spinresonance measurement. The SiO₂/Fe₂O₃ ratio by mol as used for definingthe composition of the BEA-type ferrosilicate is an expedient expressionfor defining the whole iron content including isolated iron ion Fe³⁺ inthe BEA-type ferrosilicate.

The use of small/large pore zeolite blends, particularly copperexchanged SSZ-13/Fe-BEA combinations, can increase the formation of N₂Oas compared to the constituent components. Accordingly, the use of thiscombination can be detrimental in certain applications. However, the useof copper exchanged SSZ-13/BEA-type ferrosilicate combinationssurprisingly overcame this problem and so offered improved selectivityto nitrogen. Zoned and layered SCR catalysts offer further improvements,particularly when the exhaust gas has about a 50/50 ratio of NO to NO₂.This catalyst reduces the N₂O emissions by approximately 75% over theblended equivalent whilst retaining excellent transient response andgood conversion in low NO/NO₂ gas mixes.

Surprisingly, combinations of copper exchanged SSZ-13/pre-aging Fe-BEAcan produce results substantially better than combinations haveconventional Fe-BEA. Accordingly, instead of the more conventionalprocessing of aging at 500° C. for 1 hour, the Fe-BEA material ispreferably aged at 600-900° C., preferably 650-850° C., more preferably700-800° C., and even more preferably 725-775° C., for 3-8 hours,preferably 4-6 hours, more preferably from 4.5-5.5 hours, and even morepreferably from 4.75-5.25 hours. Embodiments using copper exchangedSSZ-13/pre-aged Fe-BEA combinations are advantageous in applications inwhich the formation of N₂O is undesirable. Included within the scope ofthis invention are ratios of Cu: SSZ-13 to pre-aged Fe-BEA similar tothose of Cu:SSZ-13 to Fe-BEA and Cu:SSZ-13 to BEA-Ferrosilicate. Alsoincluded within the scope of the invention are combinations of Cu:SSZ-13to pre-aged Fe-BEA similar to those of Cu:SSZ-13 to Fe-BEA and Cu:SSZ-13to BEA-Ferrosilicate.

Both the first and second molecular sieves can be present in the samecatalyst coating, i.e. coated in a washcoat onto a suitable substratemonolith or each of the first and second molecular sieve components maybe separated in washcoat layers one above the other, with either thefirst molecular sieve component in a layer above the second molecularsieve component or vice versa. Alternatively, both the first and secondmolecular sieves can be combined in a composition for forming substratemonoliths of the extruded-type. Optionally, the extruded monolith can befurther coated with a washcoat containing one or both of the first andsecond molecular sieve component(s). Further alternatives includeforming an extruded substrate monolith comprising one, but not both, ofthe first and second molecular sieve components and coating the extrudedsubstrate monolith with a washcoat containing the other molecular sievecomponent not present in the extrudate, or the washcoat can contain boththe first and second molecular sieves. In all of the arrangementscombining extruded and coated substrate monoliths, it will be understoodthat where a first and/or a second molecular sieve component is presentin both the extrudate and the catalyst coating, the first molecularsieve component can be the same in both the extrudate and the coating,or different. So for example, the washcoat can contain SSZ-13 zeoliteand the extrudate can contain SAPO-34. The same applies for the secondmolecular sieve component, e.g. the washcoat can contain ZSM-5 zeoliteand the extrudate can contain Beta zeolite.

Washcoat compositions containing the molecular sieves for use in thepresent invention for coating onto the monolith substrate or formanufacturing extruded type substrate monoliths can comprise a binderselected from the group consisting of alumina, silica, (non zeolite)silica-alumina, naturally occurring clays, TiO₂, ZrO₂, and SnO₂.

Suitable substrate monoliths include so-called flow-through substratemonoliths (i.e. a honeycomb monolithic catalyst support structure withmany small, parallel channels running axially through the entire part)made of ceramic materials such as cordierite; or metal substrates madee.g. of fecralloy. Substrate monoliths can be filters includingwall-flow filters made from cordierite, aluminium titanate, siliconcarbide or mullite; ceramic foams; sintered metal filters or so-calledpartial filters such as those disclosed in EP 1057519 or WO 01/080978.

According to another aspect, the invention provides an exhaust systemfor treating a flowing exhaust gas containing oxides of nitrogen from amobile source of such exhaust gas, which system comprising a source ofnitrogenous reducing agent arranged upstream in a flow direction from aSCR catalyst according to the invention.

The source of nitrogenous reducing agent can comprise a suitableinjector means operated under control of e.g. a suitably programmedelectronic control unit to deliver an appropriate quantity of reducingagent or a precursor thereof (held in a suitable vessel or tank) forconverting NO_(x) to a desired degree. Liquid or solid ammonia precursorcan be urea ((NH₂)₂CO), ammonium carbonate, ammonium carbamate, ammoniumhydrogen carbonate or ammonium formate, for example. Alternatively,ammonia per se or hydrazine can be used.

In an alternative embodiment, the source of nitrogenous reducing agentis a NO_(x) absorber (also known as a NO_(x) trap, lean NO_(x) trap orNO_(x) absorber catalyst (NAC)) in combination with an engine that isconfigured so that at least one engine cylinder can operate richer thannormal operating conditions, e.g. in the remaining engine cylinders,e.g. to produce exhaust gas having a stoichiometrically balanced redoxcomposition, or a rich redox composition and/or a separate hydrocarboninjector means arranged upstream of the NO_(x) absorber for injectinghydrocarbons into a flowing exhaust gas. NO_(x) absorbed on the NO_(x)absorber is reduced to ammonia through contacting adsorbed NO_(x) withthe reducing environment. By locating the SCR catalyst according to theinvention downstream of the NO_(x) absorber, ammonia produced in situcan be utilised for NO_(x) reduction on the SCR catalyst when the NO_(x)absorber is being regenerated by contacting the NO_(x) absorber withe.g. richer exhaust gas generated by the engine.

Alternatively, the source of nitrogenous reducing agent can be aseparate catalyst e.g. a NO_(x) trap or a reforming catalyst located inan exhaust manifold of each of at least one engine cylinder which isconfigured to operate, either intermittently or continuously, richerthan normal.

In a further embodiment, an oxidation catalyst for oxidising nitrogenmonoxide in the exhaust gas to nitrogen dioxide can be located upstreamof a point of metering the nitrogenous reductant into the exhaust gasand the SCR catalyst.

The oxidation catalyst can include at least one precious metal,preferably a platinum group metal (or some combination of these), suchas platinum, palladium or rhodium, coated on a flow-through monolithsubstrate. In one embodiment, the at least one precious metal isplatinum, palladium or a combination of both platinum and palladium oran alloy of Pd—Au, optionally in combination with Pt—Pd. The preciousmetal can be supported on a high surface area washcoat component such asalumina, an aluminosilicate zeolite, silica, non-zeolite silica alumina,ceria, zirconia, titania or a mixed or composite oxide containing bothceria and zirconia.

In a further embodiment, a suitable filter substrate is located betweenthe oxidation catalyst and the catalyst according to the invention.Filter substrates can be selected from any of those mentioned above,e.g. wall flow filters. Where the filter is catalysed, e.g. with anoxidation catalyst of the kind discussed above, preferably the point ofmetering nitrogenous reductant is located between the filter and thecatalyst according to the invention. It will be appreciated that thisarrangement is disclosed in WO 99/39809. Alternatively, if the filter isuncatalysed, the means for metering nitrogenous reductant can be locatedbetween the oxidation catalyst and the filter.

In a further embodiment, the SCR catalyst for use in the presentinvention is coated on a filter or is in the form of an extruded-typecatalyst located downstream of the oxidation catalyst. Where the filterincludes the SCR catalyst for use in the present invention, the point ofmetering the nitrogenous reductant is preferably located between theoxidation catalyst and the filter.

According to a further aspect, the invention provides a lean-burninternal combustion engine comprising an exhaust system according to theinvention. In certain embodiments, the engine can be a compressionignition engine or a positive ignition engine. Positive ignition enginescan be fuelled using a variety of fuels including gasoline fuel,gasoline fuel blended with oxygenates including methanol and/or ethanol,liquid petroleum gas or compressed natural gas. Compression ignitionengines can be fuelled using diesel fuel, diesel fuel blended withnon-diesel hydrocarbons including synthetic hydrocarbons produced bygas-to-liquid methods or bio-derived components.

In yet another aspect, the invention provides a vehicle comprising alean-burn internal combustion engine according to the invention.

In a further aspect, the invention provides a method of convertingoxides of nitrogen (NO_(x)) in an exhaust gas of a mobile source whosecomposition, flow rate and temperature of which exhaust gas are eachchangeable temporally, which method comprising the step of by contactingthe NO_(x) with a nitrogenous reducing agent in the presence of aselective catalytic reduction catalyst comprising a combination of afirst molecular sieve component and a second molecular sieve component,wherein in a direct comparison tested on the Federal Test Procedure(FTP) 75 cycle the catalyst has a higher cumulative conversion of NO_(x)at equal or lower NH₃ slip than either molecular sieve component takenalone.

In one embodiment, the catalyst has a higher cumulative conversion ofNO_(x) at equal or lower NH₃ slip than either molecular sieve componenttaken alone where the cumulative molar NO:NO₂ ratio in feed gas enteringsaid catalyst is equal to, or less than 1.

In another embodiment, the catalyst has a higher cumulative conversionof NO_(x) to dinitrogen at equal or lower NH₃ slip than either molecularsieve component taken alone.

In a further embodiment, the nitrogen oxides are reduced with thenitrogenous reducing agent at a temperature of at least 100° C. Inanother embodiment, the nitrogen oxides are reduced with the reducingagent at a temperature from about 150° C. to 750° C. The latterembodiment is particularly useful for treating exhaust gases from heavyand light duty diesel engines, particularly engines comprising exhaustsystems comprising (optionally catalysed) diesel particulate filterswhich are regenerated actively, e.g. by injecting hydrocarbon into theexhaust system upstream of the filter, wherein the catalyst according tothe present invention is located downstream of the filter.

In a particular embodiment, the temperature range is from 175 to 550° C.In another embodiment, the temperature range is from 175 to 400° C.

In another embodiment, the nitrogen oxides reduction is carried out inthe presence of oxygen. In an alternative embodiment, the nitrogenoxides reduction is carried out in the absence of oxygen.

The gas containing the nitrogen oxides can contact the catalystaccording to the invention at a gas hourly space velocity of from 5,000hr⁻¹ to 500,000 hr⁻¹, optionally from 10,000 hr⁻¹ to 200,000 hr⁻¹.

The metering of the nitrogenous reducing agent contacting the SCRcatalyst can be arranged such that 60% to 200% of theoretical ammonia ispresent in exhaust gas entering the SCR catalyst calculated at 1:1NH₃/NO and 4:3 NH₃/NO₂.

In a further embodiment, a step of oxidising nitrogen monoxide in theexhaust gas to nitrogen dioxide can be performed prior to introductionof any nitrogenous reducing agent. Suitable, such NO oxidation step canbe done using a suitable oxidation catalyst. In one embodiment, theoxidation catalyst is adapted to yield a gas stream entering the SCRcatalyst having a ratio of NO to NO₂ of from about 4:1 to about 1:3 byvolume, e.g. at an exhaust gas temperature at oxidation catalyst inletof 250° C. to 450° C.

In order that the invention may be more fully understood, reference ismade to the accompanying drawings, in which:

FIG. 1 is a schematic drawing of an exhaust system embodiment accordingto the invention;

FIG. 2 is a schematic drawing of a further exhaust system embodimentaccording to the invention; and

FIG. 3 is a graph showing the results of NO_(x) conversion activitytests described in Example 3 on fresh catalysts prepared according toExamples 1, 2 and 3.

FIGS. 4a-4d shows different types of combinations of a first molecularsieve and a second molecular sieve on a substrate.

FIGS. 5, 6 a, 6 b, 7, and 8 a-8 c are graphs showing data associatedwith certain embodiments of the invention.

In FIG. 1 is shown an apparatus 10 comprising a light-duty diesel engine12 and an exhaust system 14 comprising a conduit for conveying exhaustgas emitted from the engine to atmosphere 15 disposed in which conduitis a metal substrate monolith coated with a NO_(x) Absorber Catalyst((NAC)) also known as a NO_(x) trap or lean NO_(x) trap) 16 followed inthe flow direction by a wall-flow filter 18 coated with a SCR catalystaccording to the invention (Cu/SSZ-13 blended with an iron-in-zeoliteframework BEA also ion-exchanged with additional ion-exchanged iron). Aclean-up catalyst 24 comprising a relatively low loading of Pt onalumina is disposed downstream of wall-flow filter 18.

In use, the engine runs lean of stoichiometric, wherein NO_(x) isabsorbed in the NAC. Intermittently, the engine is run rich to desorband reduce NO_(x). During rich running operation, some NO_(x) is reducedto NH₃ and is stored on the downstream SCR catalyst for further NO_(x)reduction. The SCR catalyst also treats NO_(x) during intermittent richevents. NO oxidised to NO₂ on the NAC is used to combust soot trapped onthe filter 18 passively. The NAC is also used to combust additionalhydrocarbon during occasional forced (active) regenerations of thefilter.

FIG. 2 shows an alternative apparatus 11 according to the inventioncomprising a diesel engine 12 and an exhaust system 13 therefor. Exhaustsystem 13 comprises a conduit 17 linking catalytic aftertreatmentcomponents, namely a 2Au-0.5Pd/Al₂O₃ catalyst coated onto an inertceramic flow-through substrate 19 disposed close to the exhaust manifoldof the engine (the so-called close coupled position). Downstream of theclose-coupled catalyst 19, in the so-called underfloor position, is anflow-through catalyst 22 of the extruded type comprising a mixture of analuminosilicate CHA ion-exchanged with Cu and FeCHA, having Fe presentin the molecular sieve framework structure. A source of nitrogenousreductant (urea) is provided in tank 28, which is injected into theexhaust gas conduit 17 between catalysts 19 and 22.

In certain embodiments, provided is a catalyst for selectivelycatalysing the conversion of oxides of nitrogen using a nitrogenousreductant in a feed gas whose composition, flow rate and temperature areeach changeable temporally, which catalyst comprising a combination of afirst molecular sieve component and a second molecular sieve component,wherein in a direct comparison tested on the Federal Test Procedure(FTP) 75 cycle the catalyst has a higher cumulative conversion of NO_(x)at equal or lower NH₃ slip than either molecular sieve component takenalone.

Preferably, the catalyst has a higher cumulative conversion of NO_(x),preferably to elemental nitrogen, at equal or lower NH₃ slip than eithermolecular sieve component taken alone where the cumulative molar NO:NO₂ratio in feed gas entering said catalyst is equal to, or less than 1.

In certain embodiments, the SCR catalyst has the first molecular sievecomponent achieves the maximum NO_(x) conversion at a lower NH₃ filllevel for the conditions selected than the second molecular sievecomponent. Preferably, the lower NH₃ fill level of the first molecularsieve component is in the range of 10-80%.

In certain embodiments in the SCR catalyst, the first and secondmolecular sieves can be selected independently from zeolites andnon-zeolite molecular sieves. Preferably, one of the molecular sievecomponents is a small pore molecular sieve containing a maximum ringsize of eight (8) tetrahedral atoms, preferably selected from the groupof Framework Type Codes consisting of: ACO, AEI, AEN, AFN, AFT, AFX,ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO,IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT,SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON, with CHA, LEV, ERI, DDR,KFI, EAB, PAU, MER, AEI, GOO, YUG, GIS and VNI being particularlypreferred. Preferably, the other molecular sieve component is selectedfrom a small pore, medium pore, large pore or meso-pore size molecularsieve. Preferred medium pore molecular sieve include MFI, MWW, AEL, AFO,FER and HEU. Preferred large pore molecular sieve include FAU, AFI, AFR,ATO, BEA, GME, MOR and OFF.

In certain embodiments, one or both of the first and second molecularsieves contains a substituent framework metal selected from the groupconsisting of As, B, Be, Co, Fe, Ga, Ge, Li, Mg, Mn, Ti, Zn and Zr. Incertain embodiments, one or both of the first molecular sieve componentand the second molecular sieve component contain one or more metalselected independently from the group consisting of Cr, Mn, Fe, Co, Ce,Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Ir and Pt, preferablyCr, Ce, Mn, Fe, Co, Ni and Cu.

In one aspect of the invention, provided is an exhaust system fortreating a flowing exhaust gas containing oxides of nitrogen from amobile source of such exhaust gas, which system comprising a source ofnitrogenous reducing agent arranged upstream in a flow direction from aselective catalytic reduction catalyst described herein. In certainembodiments, the system further comprises an oxidation catalyst disposedupstream of the source of nitrogenous reducing agent and the SCRcatalyst. In certain embodiments, the system further comprises a filterdisposed between the oxidation catalyst and the source of nitrogenousreducing agent.

In one aspect of the invention, provided is a lean-burn internalcombustion engine, such as a compression ignition engine or a positiveignition engine, comprising an exhaust system described herein. Incertain embodiments, the engine comprises a NO_(x) absorber whichfunctions, at least in part, as the source of a nitrogenous reducingagent.

In one aspect of the invention, provided is a vehicle comprising alean-burn internal combustion engine described herein.

In one aspect of the invention, provided is a method for convertingoxides of nitrogen (NO_(x)) in an exhaust gas of a mobile source thecomposition, flow rate and temperature of which exhaust gas are eachchangeable temporally, which method comprising the step of contactingthe NO_(x) with a nitrogenous reducing agent in the presence of aselective catalytic reduction catalyst comprising a combination of afirst molecular sieve component and a second molecular sieve component,wherein in a direct comparison tested on the Federal Test Procedure(FTP) 75 cycle the catalyst has a higher cumulative conversion of NO_(x)at equal or lower NH₃ slip than either molecular sieve component takenalone. In certain embodiments of the method, the catalyst has a highercumulative conversion of NO_(x), preferably to dinitrogen, at equal orlower NH₃ slip than either molecular sieve component taken alone wherethe cumulative molar NO:NO₂ ratio in feed gas entering said catalyst isequal to, or less than 1. In certain embodiments of the method, theNO_(x) is converted at a temperature of at least 100° C., preferablyfrom about 150° C. to 750° C. In certain embodiments of the method, thegas containing the NO_(x) contacts the SCR catalyst at a gas hourlyspace velocity of from 5,000 hr⁻¹ to 500,000 hr⁻¹. In certainembodiments, about 60% to about 200% of theoretical ammonia contacts theSCR catalyst calculated at 1:1 NH₃/NO and 4:3 NH₃/NO₂. In certainembodiments of the method, the NO:NO₂ ratio in gas contacting the SCRcatalyst is from about 4:1 to about 1:3 by volume.

The following Examples are provided by way of illustration only.

EXAMPLES Example 1—Method of Making Fresh 3 wt % Cu/SSZ-13(Aluminosilicate CHA) Catalysts

Commercially available SSZ-13 zeolite (CHA) was NH₄ ⁻ ion exchanged in asolution of NH₄NO₃, then filtered. The resulting materials were added toan aqueous solution of Cu(NO₃)₂ with stirring. The slurry was filtered,then washed and dried. The procedure can be repeated to achieve adesired metal loading. The final product was calcined. The materialsprepared according to this Example are referred to herein as “fresh”.

Example 2—Method of Making Fresh 5 wt % Fe/Beta Catalyst

Commercially available Beta zeolite was NH₄ ⁺ ion exchanged in asolution of NH₄NO₃, then filtered. The resulting material was added toan aqueous solution of Fe(NO₃)₃ with stirring. The slurry was filtered,then washed and dried. The procedure can be repeated to achieve adesired metal loading. The final product was calcined. The materialsprepared according to this Example are referred to herein as “fresh”.

Example 3—Catalyst Mixtures

Separate physical blends of fresh and aged 1:3 Fe/Beta:Cu/SSZ-13 byweight were prepared by physical mixture of samples made according toExamples 1 and 2. Likewise physical blends of 1:3BEA-Ferrosilicate:CuSSZ-13 by weight were prepared.

Example 4—NO_(x) Conversion Activity Tests

The activity of the fresh powder samples prepared according to Examples1, 2 and 3 were tested at 250° C. in a laboratory apparatus using thefollowing gas mixture: 125 ppm NO, 375 ppm NO₂ 750 ppm NH₃, 14% O₂, 4.5%H₂O, 4.5% CO₂, N₂ balance at a space velocity of 60,000 hr⁻¹. The testis stopped when 20 ppm NH₃ is detected downstream of the sample. Theresults are shown in FIG. 3.

From the results it can be seen that the Fe/Beta sample has a fasttransient response, but limited maximum conversion. It also slips NH₃early on in the test compared with the Cu/SSZ-13 and Fe/Beta+Cu/SSZ-13blend. Transient response is defined as the rate at which NOx conversionincreases as the level of NH₃ fill on the catalyst increases. TheCu/SSZ-13 has better, higher maximum conversion but a slower transientresponse. The combination of Fe/Beta and Cu/SSZ-13 gives fast transientresponse, higher maximum conversion, but also has higher conversion thanthe individual components at intermediate NH₃ fill levels, which isevidence of synergy. Pre-aged 1:3 Fe/Beta:Cu/SSZ-13 will provideimproved results as well.

Example 5—Comparison of Blends, Layers, and Zones Combinations

Three samples of a 1:3 (by weight) BEA-Ferrosilicate:CuSSZ-13combination were prepared and separately coated on substrates as ablend, zones, and layers. The three coated substrates were exposed to atest environment similar to that described in Example 4, except that theNO:NO₂ ration was about 50:50. The results are shown in FIG. 5.

From the results it can be seen that zones and blends achieve higher NOxconversion compared to blends.

Example 6—N₂O Formation

A samples of a 1:3 (by weight) FeBEA:CuSSZ-13 combination and threesamples of a 1:3 (by weight) BEA-Ferrosilicate:CuSSZ-13 combination wereprepared and separately coated on substrates. The FeBEA:CuSSZ-13combination was coated as a blend, whereas the samples ofBEA-Ferrosilicate:CuSSZ-13 combination were separately coated as ablend, zones, and layers. Each of the samples were exposed to asimulated diesel gas exhaust combined with NH₃ dosing (20 ppm slip). Theaverage N₂O formation during exposure was recorded and is shown in FIGS.6a and 6 b.

It is clear that the FeBEA:CuSSZ-13 blend produces significant N₂Oresulting in an apparent reduction in maximum conversion and ‘N₂selective’ transient response. This reduction in conversion alsooutweighs that observed for the two components evaluated independently.Surprisingly, the BEA-Ferrosilicate:CuSSZ-13 blend producessubstantially less N₂O than that observed for any other CuSSZ-13/zeoliteblend. However, layers and zones of BEA-Ferrosilicate:CuSSZ-13 maintainthe low N₂O make observed for the blend, but also show improvedtransient response under different NO₂ levels (see FIG. 7).

Example 7—Effect of NO:NO2 Ratios

Four samples of BEA-Ferrosilicate:CuSSZ-13 were prepared and tested forNOx conversion capacity during exposure to simulated diesel exhaust gascombined with a NH₃ reductant. Testing was performed at 250° C. and gashourly space velocity of about 60,000/hour. The results are provided inthe table below. Here, the reference (“ref.”) catalyst is CuSSZ-13, “lowfill” refers to an NH₃ level at less than about 0.5 g/L of exhaust gas,and “high fill” refers to an NH₃ at greater about 1 g/L of exhaust gas.

0% NO₂ 50% NO₂ 75% NO₂ 3:1 BEA-Ferro- Better than ref Better than refSimilar at low silicate:CuSSZ- fills, better at 13 (BLEND) high fills1:1 BEA-Ferro- Poor at low fills, Better than ref Similar at lowsilicate:CuSSZ- better at high fills, better at 13 (BLEND) fills thanref high fills 1:3 BEA-Ferro- Similar to ref Much better Better than refsilicate:CuSSZ- than ref. Much at low fills, 13 (ZONE) betterselectivity similar at high fill 1:3 BEA-Ferro- Similar to ref Muchbetter Better than ref silicate than ref. Much at low fills, layeredover better selectivity similar at high CuSSZ-13 fill

Example 8—Multiple Combinations

Samples of FeBEA, CuSSZ-13, and BEA-Ferrosilicate were prepared cancombined in the indicated combinations and multiple combinations shownin FIGS. 8a-8c . In the legends, the ratios are give by weight, blendsare shown in parenthesises, and zones are indicated by “//”, with thefirst named component disposed upstream with respect to gas flow pastthe catalyst. Each of the combinations and multiple combinations wereexposed to a simulated diesel gas exhaust gas stream containing an NH₃reductant. The NO:NO₂ ratio in the exhaust gas was varied from only NO,50:50 NO:NO₂ (by weight), and 75% NO₂ (by weight), to test the catalystat different conditions. Each combination or multiple combination wasevaluated for NO_(x) conversion (corrected for N₂O formation) as afunction of NH₃ fill level.

The invention claimed is:
 1. A catalyst composition for treating exhaustgas comprising: (a) a first molecular sieve having BEA framework,wherein said first molecular sieve is a ferrosilicate; and (b) a secondmolecular sieve having a small pore crystal structure and containingabout 0.5 to about 5 weight percent of exchanged or free metal selectedfrom Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Pt, Ag, In, Sn,Re, and Ir, wherein the second molecular sieve is a zeolitealuminosilicate having a silica-to-alumina ratio of about 8 to about150, wherein said first molecular sieve and said second molecular sieveare present in a weight ratio of about 0.1 to about 1.0.
 2. The catalystcomposition of claim 1, wherein said exchanged or free metal in thesecond molecular sieve is copper.
 3. The catalyst composition of claim2, wherein said second molecular sieve has a framework selected fromCHA, AEI, AFX, and LEV.
 4. The catalyst composition of claim 1, whereinsaid second molecular sieve contains about 2.0 to about 4.0 weightpercent copper, and said first molecular sieve and said second molecularsieve are present in a weight ratio of about 0.2 to about 0.60.
 5. Thecatalyst of claim 4, wherein the second molecular sieve has a CHAframework.
 6. The catalyst of claim 4, wherein the second molecularsieve has an AEI framework.
 7. The catalyst of claim 4, wherein thesecond molecular sieve has an AFX framework.
 8. The catalyst of claim 4,wherein the second molecular sieve has a LEV framework.
 9. A catalystarticle comprising: (a) a catalyst composition according to claim 1; and(b) a monolith substrate onto or within which said catalytic compositionis incorporated, wherein said first and second molecular sieves arepresent as a blend, a plurality of layers, or a plurality of zones. 10.The catalyst article of claim 9, wherein said first molecular sieve isdisposed in a first zone and said second molecular sieve is disposed ina second zone, and wherein at least a portion of said first zone isupstream of said second zone relative to an intended direction of gasflow past or through said catalytic composition, a majority of saidfirst zone does not overlap said second zone, and a majority of saidsecond zone does not overlap said first zone.
 11. The catalyst articleof claim 9, wherein said first molecular sieve is disposed in a firstzone and said second molecular sieve is disposed in a second zone, andwherein said second zone is disposed below said first zone relative toan intended direction of gas flow past or through said catalyticcomposition.
 12. The catalyst article of claim 9, wherein said first andsecond molecular sieves are disposed on said substrate as a blend.